Liquid crystal display devices have become rapidly in widespread use, instead of conventional CRT displays, because of their features such as low power consumption, lightweight properties, and thin profiles.
A common display device has, as illustrated in FIG. 2, for example, a back light source (not illustrated), a polarizer 25 on the back light side, a liquid crystal cell 21, a color filter 22, and a polarizer 23 on the screen side. The polarizers 23 and 25 each selectively transmit linearly polarized light having a vibrating surface in a specific vibrating direction only, and are arranged to face each other in a cross-Nicol configuration so that the vibrating directions are orthogonal to each other. The liquid crystal cell 21 is placed between the polarizers 23 and 25.
Polarizers used in a liquid crystal display device commonly have polarizer protective films. Films formed of cellulose esters typically such as triacetylcellulose have been commonly used as the polarizer protective films. Such use is based on an advantage that cellulose esters hardly influence the display quality because of their low retardation values. In addition, the use is also based on an advantage that moistures left in a polarizing device in production of a polarizer can be dried through the polarizer protective film owing to appropriate permeability of cellulose esters. Moreover, comparative inexpensiveness of cellulose ester films also contributes to the use.
Various problems, however, are left when such cellulose ester films are considered as a material supporting a liquid crystal display device industry that will continue to expand. Particularly serious problems are mentioned below.
First, production of cellulose ester films commonly employs a so-called solution casting method including the steps of casting a solution containing a cellulose ester dissolved in an organic solvent on a supporting member, drying the solvent, and peeling the dried product to give a film. Such solution casting method requires large-scale equipment and special techniques for the solvent drying step and the like. Accordingly, only manufacturers having special techniques can produce the films, failing to meet the demand from an expanding market of liquid crystal display devices. In the future, too-much dependence on the cellulose ester films as polarizer protective films may disturb development of the liquid crystal display device industry that is now a major industry of our country.
Additionally, in production of cellulose ester films by the solution casting method, dichloromethane is commonly used as a main solvent in an organic solvent used in a cellulose ester solution. The dichloromethane is, however, suspected of being harmful to human bodies. Accordingly, dependence on cellulose ester films in future increases the consumption and the discharge of dichloromethane along with the development of the liquid crystal display device industry, which is environmentally undesirable.
To solve such problems of cellulose ester films, use of a general-purpose film that is commercially available or easily produced by a simple method, as a polarizer protective film has been desired. For example, use of polyester films such as polyethylene terephthalate films, instead of cellulose ester films, is now tested (see Patent Literature 1).